1. Field of the Invention
The present invention relates to a communication system and, more particularly, to a communication system which uses a wavelength division multiplexing optical network.
2. Description of the Related Art
With the progress of information-intensive society, a backbone network that permits higher communication traffic is in increasing demand. As a method of implementing such a backbone network there is a wave length division multiplexing system in which light signals of different wavelengths individually modulated with data propagate through an optical fiber. So far, several methods of implementing a backbone network using wavelength division multiplexing have been proposed (for example, see Institute of Electronic Information and Communications Engineers Technical Report-Optical Communications System OCS92-77 to 82).
Of these methods, the method by Shimosaka et al. (OCS92-79) splits each wavelength into short duration time slots, a transmitter groups data into packets for each destination and sends them in an empty time slot. That is, this method uses both the wavelength division multiplexing and the time division multiplexing in combination. This method is effective for a system such as a backbone LAN (local area network), in which requests for transmission to many other stations are generated at almost the same time.
That system by Shimosaka et al. is a fixed receiving wavelength system, which sets a transmitting wave length of an optical transmitter of a source to a receiving wavelength of a destination. In this system, any amount of data can be transmitted as long as there is an empty time slot or slots for a reception node. If, therefore, node A uses all of empty time slots of node B, then node C located between node A and node B cannot send data to node B even though it desires to send. That is, node C has to wait for sending until node A finishes sending data to node B. This would result in a significant increase in the delay or discard rate. Also, a bandwidth for a connection oriented call cannot be guaranteed. In a backbone network for interconnecting LANs, requests for sending many nodes occur at almost the same time. Further, a number of connection oriented calls such as telephone, video, etc. are generated. Thus, such network as described above in which the network traffic is limited by particular nodes (in the above, node A) and the network bandwidth cannot be guaranteed is not suitable for backbone networks.
For a LAN, a broadcast function for sending the same data to all of nodes and a multicast function for sending the same data to an arbitrary number of nodes are important. Hereinafter, the broadcast and multicast are generically named "broadcast/multicast". To implement the broadcast/multicast functions by that system by Shimosaka et al, which is of a fixed receiving wavelength type, data need to be copied for subsequent transmission to all of reception nodes, which results in reduced efficiency.
Even if the receiving wavelength is made variable, if each node is equipped with a single data receiver, data receivers at reception nodes for broadcast/multicast must be ready to receive data at the timing of a transmission time in broadcast/multicast communications, that is, they must be placed in the state where they are receiving no data from other than a broadcasting/multicasting node. In other words, to permit broadcast/multicast communications when each node has a single data transmitter and receiver, there must be an empty time slot that is common to the data receivers. Up to now, however, how to implement such a state has not been considered. As described in, for example, Institute of Electronic Information And Communications Engineers Technical Research Report-Optical Communications System OCS92-84, that problem will be solved by equipping each node with multiple data transmitters and receivers. However, this approach will significantly increase the hardware cost and is not thus desirable in terms of implementation and cost-performance of a system.
As described above, problems with the optical communication system that uses conventional wavelength division multiplexing and time division multiplexing at the same time are that: (1) since the traffic is limited by part of nodes which transmit a large quantity of data, other nodes are not allowed to send data or the bandwidth of the call during transmitting is not guaranteed, and (2) even if an attempt is made to perform the broadcast/multicast facilities, an empty time slot common to all the reception nodes is not available, failing to perform broadcast/multicast communications.